Operating medium for carbon dioxide-cooling systems and air-conditioning systems

ABSTRACT

The invention relates to resource operating medium compositions containing additivated lubricants based on polyalkylene glycols and/or neopentyl polyol esters, which are additivated with alkylated triaryl phosphate esters which are suitable for lubricating refrigerators, air-conditioning systems, heat pumps and similar systems which are operated using carbon dioxide as an operating medium.

The invention relates to operating agent compositions comprising addedlubricants based on polyalkylene glycols and/or neopentyl polyol esterssuitable for lubricating refrigerating machines, air conditioningequipment, heat pumps and related equipment operated with carbon dioxideas refrigerant.

Carbon dioxide was used as an operating agent for refrigerating machinesas early as at the beginning of modern cryo-engineering. Thus, Lindebuilt the first compression refrigerating machine as early as in 1881using carbon dioxide as refrigerant. Up to the middle of this century,carbon dioxide was used mainly in ship refrigerating equipment withsub-critical process controls. Glycerine was used as lubricant. Lateron, following the introduction of fluorochlorohydrocarbon refrigerants,carbon dioxide was hardly used.

At present, the halogenated fluorohydrocarbon R134a is mainly used inmotor vehicle air conditioning equipment and refrigerant mixtures suchas R404A are used in frozen food equipment. The use of theold-established refrigerant carbon dioxide (R744) is being reconsideredin recent years on an increasing scale. Polyalkylene glycols (PAG) havealready been suggested for use as lubricants in motor car airconditioning systems (compare e.g. ‘Polyalkylenether-Schmierstoffe fürCO₂-Pkw-Klima-Systeme’—polyalkylene ether lubricants for CO₂ motor carair conditioning systems, J. Fahl, E Weidner in Luft- and Kältetechnik36 (2000) 10, page 478-481, ISSN 0945-0459). Polyol esters have beensuggested for use in CO₂ frozen food equipment (compare e.g. Esterölefür CO₂-Kälte- und Klimasysteme—ester oils for CO₂ refrigerating and airconditioning systems, J Fahl in Kälte—and Klimatechnik 53 (2000) 11,page 38-45, ISSN 0343-2246).

The advantages of the natural working material carbon dioxide (CO₂) canbe exploited in a trans-critical cyclic process; however, considerablyhigher operating pressures occur in this case than would correspond tothe present state of the art. In such a cycle, the operating medium ispresent both in the sub-critical and in the super-critical state andpreviously unknown lubrication problems arise. On the one hand, almostcomplete miscibility between the lubricating oil and CO₂ is required attemperatures going as low −40° C.; on the other hand, correspondinglubrication and stability properties need to be guaranteed under theinfluence of CO₂ at pressures of up to 150 bar and temperatures of up to220° C. In air conditioning equipment, in particular, the lubricatingoil is subject to extreme mechanical and thermal stresses. Tribologicaldifficulties occur in test compressors of the most varied types ofdesign.

The main cause of compressor failure has initially been assumed to bethe comparatively high CO₂-solubility in lubricating oil and theresulting dilution and degasification effects. Initial practicalinvestigations in piston compressors operated at sub-critical level haveshown that, in spite of maintaining the minimum required mixtureviscosity, extreme wear phenomena occur as a result of the effect of CO₂which phenomena are attributable to mixed friction and lack oflubrication. In the first prototype compressors of motor car airconditioning systems operated at the trans-critical level, lubricationproblems were observed when commercial neopentyl polyol esters (POE) orpolyalkylene glycol (PAG) oils were used.

Only oils of certain chemical compounds exhibit the necessary propertiessuch as e.g. a correspondingly satisfactory cold flow behaviour and afavourable solubility with CO₂. Investigations have shown that thephysical properties and the interactions between different basic oilsand sub-critical and super-critical CO₂ depend to a large extent ontheir chemical composition. Mineral oils are almost immiscible with CO₂and, as a result of the rather moderate high temperatures stabilitycompared with synthesis oils, have proved to be hardly suitable. Becauseof their unfavourable phase behaviour and the comparatively low density,in particular, both hydrocracking oils and alkyl aromatics as well aspolyalphaolefins (PAO) must be classified as unsuitable for use insystems with a battery on the intake side.

As a result of the comparatively high volumetric refrigeration output ofCO₂ and the increased efficiency, cryogenic compressors can bedimensioned smaller for carbon dioxide. This requires a high loadcarrying capacity of the lubricant in the corresponding temperaturerange.

Practical experience has shown that polyalkylene glycols possessexcellent friction properties. The satisfactory absorption to metalsurfaces can be attributed to the polar character. As a result of thishigh surface activity and the low viscosity pressure dependence, lowfriction coefficients are achieved.

In the tribological contact areas subject to the influence of CO₂,special conditions are present. At the moment of start up and shut down,in particular, strong solubility-dependent effects occur which inhibitthe formation of a sufficient lubrication film thus allowing theclearance filled by oil film to be washed out as a result of dissolvedrefrigerant, the washing out being caused, among other things, by thepressure equalisation and the changes in surface tension occurring. Wearmeasurements on prototype compressors of different design have shown,however, that the dilution and degasification effects described can becompensated for only to some extent by using correspondingly highlyviscous oils. In this respect, sufficient oil recycling from theevaporator is not always guaranteed. Moreover, the investigationscarried out with piston compressors operated at sub-critical level haveindicated that, in spite of a sufficiently high mixture viscosity, anunusually high stress is present in the area of mixed friction ispresent. Since, in practical tribotechnological systems, asuperimposition of the different elemental wear mechanisms usuallyoccurs, the wear behaviour cannot be assessed theoretically but can bedetermined only experimentally by corresponding wear tests.

From the purely tribological point of view, only little CO₂ shoulddissolve in the refrigerating machine oil as far as possible. On theother hand, a satisfactory miscibility is required for oil recycling andthe heat transfer in the cold cycle.

The invention is consequently based on the problem of adding lubricantsfor carbon dioxide refrigerants in a suitable manner such that themixture of carbon dioxide and lubricant satisfies the followingrequirements, apart from those mentioned above:

-   -   excellent lubrication properties and a high load carrying        capacity    -   optimal anti-seizure performance and mixed friction conditions    -   excellent thermal and chemical stability.

For serious stress conditions in cryogenic CO₂ compressors, the personsskilled in the art has access to the use of standard known anti-wearadditives and/or high pressure additives. The anti-wear additivesgenerally used in the lubricant sector are based on organometalcompounds such as zinc/phosphorus or zinc/sulphur compounds such as zincdithiophosphate (ZDTP). The usual active, low ash agents, on the otherhand, comprise no metallic elements and consist e.g. of organicmonosulphides and polysulphides, saturated and unsaturated fatty acids,natural and synthetic fatty esters and primary and secondary alcohols.

Surprisingly enough, certain additives and basic oil combinations haveproved suitable for solving the above-mention tasks:

Operating agent compositions for refrigerating machines, heat pumps andallied equipment such as air conditioning equipment comprising

-   -   (A) Carbon dioxide as refrigerant, the refrigerant preferably        consisting essentially exclusively of carbon dioxide,    -   (B) A polyalkylene glycol and/or a neopentyl polyol ester as        lubricant and    -   (C) A phosphate ester with the following structure as additive:

wherein

-   -   R optionally, identically or differently for each of the three        phenyl moieties and optionally, identically or differently for        each n, represents H or one or more C1 to C6 hydrocarbon        moieties and    -   n optionally identically or differently for each of the three        phenyl moieties represents an integer of 1 to 5, preferably 1, 2        or 3 with the proviso that for at least one of the three phenyl        moieties    -   R is a C2 to C6 hydrocarbon preferably t-butyl and/or isopropyl.

Preferred embodiments of the above operating agent composition are thesubject matter of the sub-claims and/or will be explained in thefollowing.

Additives

The phosphate ester tricresyl phosphate which is known as a lubricantadditive is not a subject matter of the invention (compare Tables 2 and3, and the reference example). Tricresyl phosphate is a mixture ofphosphates ortho-substituted, para-substituted or meta-monomethylsubstituted at the phenyl ring.

The phosphate ester used according to the invention is preferably usedin a quantity of 0.1 to 3% by weight, particularly preferably 0.3 to1.5% by weight, based on the lubricant.

T-butylated triphenyl phosphates are usually produced by the alkylationof phenols and reaction with phosphoric acid trichloride. According to apreferred variation, the phosphate esters used according to theinvention exhibit at least one phenyl moiety alkylated in theortho-position.

In comparison with additives with added sulphur or chlorinatedadditives, the claimed triaryl phosphates are less reactive and have theadvantage that they cause neither corrosion nor discolouration in thecase of most metals. Moreover, these active substances highly soluble inthe claimed basic oils are characterised by their extraordinarily highthermal and oxidative stability.

In contrast to anti-wear additives comprising sulphur and zinc, theclaimed phosphates are considerably more stable under the influence ofCO₂ and allow high application temperatures to be used. In particular,t-butylated triphyl phosphates are characterised by a very highhydrolytic stability.

Polyalkylene Glycols

The polyalkylene glycols (PAG) used according to the invention exhibitalkylene oxide units with 1 to 6 carbon atoms (—R—O—) as monomer units.

The polyalkylene glycols exhibit hydrogen end groups, alkyl, aryl,alkylaryl, aryloxy, alkoxy, alkylaruloxy and/or hydroxy end groups.Alkylaryloxy groups should also be understood to mean arylalkyl (ene)oxygroups and alkylaryl groups to mean arylalkyl(ene) groups (e.g. arylCH₂CH₂—). The end groups of the alkyl type, including the alkoxy type,or of the aryl types, including the alkylaryl type, aryloxy type andalkylaryloxy type preferably exhibit 6 to 24 carbon atoms, particularlypreferably 6 to 18 carbon atoms, based on the aryl types, and preferably1 to 12 carbon atoms, based on the alkyl types.

The polyalkylene glycols according to the invention are consequentlyeither homopolymers, namely polypropylene glycol (and/or polypropyleneoxide) or copolymers, terpolymers etc. For the latter cases, the monomerunits may exhibit a random distribution or a block structure. If thepolyalkylene glycols are not homopolymers, preferably at least 20%,preferably at least 40% of all monomer units are producible frompolypropylene oxide (PO), and also preferably, at least 20% of allmonomer units of these polyalkylene glycols are producible by usingethylene oxide (EO) (PO/EO copolymers).

According to a further embodiment, preferably at least 20%, preferablyat least 40% of all monomer units are producible from butylene oxide(BO) and, moreover, preferably at least 20% of all monomer units ofthese polyalkylene glycols are producible by using ethylene oxide (BO/EOcopolymers).

When (poly)alcohols are used, the starting compound is incorporated intothe polymer and, according to the meaning of the invention, alsoreferred to as end group of the polymer chain. Suitable starting groupsconsist of compounds comprising active hydrogen such as e.g. n-butanol,propylene glycol, ethylene glycol, neopentyl glycols such aspentaerythritol, ethylene diamine, phenol, cresol or other (C1 to C16(mono, di or tri)alkyl) aromatics, (hydroxyalkyl) aromatics,hydroquinone, aminoethanolamines, triethylenetetramines, polyamines,sorbitol or other sugars. Other C—H acidic compounds such as carboxylicacids or carboxylic anhydrides can also be used as starting compounds.

Preferably, the polyalkylene glycols comprise aryl groups orcorresponding heteroaromatic groups, e.g. inserted into the polymerchain, as side groups or end groups; the groups may, if necessary, besubstituted with linear or branched alkyl groups or alkylene groups, thealkyl groups or alkylene groups overall exhibiting preferably 1 to 18carbon atoms. Suitable polyalkylene glycols are possibly producible byusing the corresponding starting alcohol compounds, e.g. of thefollowing type:

in which x and y represent an integer of 0 to 6, x+y is less than 7, x+yis more than 1 and y is either greater than 0 (preferably 1 to 3) or R¹carries one or several hydroxy groups. It is also possible for y to begreater than 0 and R¹ to carry one or several hydroxy groupssimultaneously. Preferably, y is an integer of 1 to 3. R¹ represents alinear or branched C¹ to C18 hydrocarbon group which, if necessary,carries one or several hydroxy groups. The starting alcohol compound mayalso, in the same way, consist of a condensed aromatic such asnaphthalene instead of benzene.

Cyclic ether alcohols such as hydroxyfurfuryl or hydroxytetrahydrofuran,nitrogen heterocyclics or sulphur heterocyclics can also be used asstarting groups. Such polyalkylene glycols are disclosed in WO 01/57164which is herewith also made part of the subject matter of thisapplication.

Preferably, the polyalkylene glycols according to the invention have anaverage molecular weight (number average) of 200 to 3000 g/mole,particularly preferably 400 to 2000 g/mole. The kinematic viscosity ofthe polyalkylene glycols is preferably 10 to 400 mm²/s (cSt) measured at40° C. according to DIN 51562.

The polyalkylene glycols used according to the invention can be producedby reacting alcohols, including polyalcohols, as starting compounds withoxiranes such as ethylene oxide, propylene oxide and/or butylene oxide.Following the reaction, these possess only one free hydroxy group as endgroup. Polyalkylene glycols with only one hydroxy group are preferredover those with two free hydroxy groups. Polyalkylene glycols which,e.g. after a further etherification step, comprise no free hydroxygroups any longer are particularly preferred regarding the stability,hygroscopicity and compatibility. The alkylation of terminal hydroxylgroups leads to an increase in the thermal stability and an improvementin the CO₂ miscibility.

By selecting suitable end groups, the miscibility can, moreover, beadjusted in such a way that, in the phase diagram of T against a portionof lubricant in CO₂, areas of complete miscibility exist and those withno or only a slight miscibility.

Neopentyl Polyolesters and Lubricant Mixtures

It is also possible to use neopentyl polyolesters, if necessary togetherwith the polyalkylene glycols described above, in the operating agentsaccording to the invention.

The esters of neopentyl polyols such as neopentyl glycol,pentaerythritol and trimethylol propane with linear or branched C4 toC12 monocarboxylic acids, e.g. with addition of correspondingdicarboxylic acids are suitable neopentyl polyolesters. Usually,pentaerythritol is obtainable as technical grade pentaerythritol whichis a mixture of monopentaerythritol, dipentaerythritol andtripentaerythritol. However, their condensation products such asdipentaerythritol and/or tripentaerythritol are also suitable as alcoholcomponents.

Pentaerythritol or mixtures with dipentaerythritol and/ortripentaerythritol, preferably mixtures comprising predominantlydipentaerythritol are particularly suitable.

Complex esters can be produced by proportional esterification ofpolyhydric alcohols with monovalent and divalent acids such as C₄ to C₁₂dicarboxylic acids. In this way, dimers and oligomers are formed. Whenusing neopentyl glycol and/or trimethylol propane as alcohol group,complex esters are preferred.

In the test stand test described in the experimental part, thephosphoric acid esters used according to the invention have,surprisingly enough, proved to be excellent additives for improving thelubrication effect of the neopentyl polyol esters when used togetherwith carbon dioxide as refrigerating machine operating agent, even whenthese neopentyl polyol esters are used as such, i.e. without usingpolyalkylene glycols. Neopentyl polyol esters have been regarded so faras being less suitable for use together with carbon dioxide as operatingagent in refrigerating machines because of their less satisfactorylubrication properties—in comparison with polyalkylene glycols.

Compounds obtainable from neopentyl polyols and carboxylic acids arereferred to as neopentyl polyol esters. Polyols not exhibiting hydrogenatoms in position β to the hydroxy group are referred to as neopentylpolyols. These are polyols with preferably 2 to 8 hydroxy groups, one,two or three quaternary carbon atoms and 5 to 21, preferably 5 to 15carbon atoms, the hydroxy groups of the polyol, as alcohol component,being coupled only with those carbon atoms which, in turn, exhibit onlyquaternary carbon atoms in the vicinal position.

Examples of these are neopentyl polyol (NPG), trimethylol propane (TMP),pentaerythritol (PE). Neopentyl polyols as alcohol component maycomprise, moreover, 1 to 4 ether bridges. The alcohol componentpentaerythritol and/or dipentaerythritol (DPE) and/or tripentaerythritol(TPE) is particularly preferred.

Preferred acid components consist of n-pentanoic acid, n-heptanoic acid,octanoic acid, decanoic acid, 2-ethyl hexanoic acid, 3,5,5-trimethylhexanoic acid and 2-hexyl decanoic acid as well as other Guerbet acidsor their mixtures. To produce complex esters, adipic and dodecane dioicacid are particularly suitable. It has proved advantageous to producethe neopentyl polyol esters by reacting the corresponding alcohols withmixtures of the corresponding acids. The complete esterification of allhydroxy groups of the neopentyl polyols and acid groups of thedicarboxylic acids, which may be used if necessary, is preferred.

According to a further variant of the invention, the polyalkyleneglycols used according to the invention can be employed together withneopentyl polyol esters as lubricants. Regarding the definition of thepreferred alcohol groups of these neopentyl polyol esters, referenceshould be made to the above paragraphs.

Further Additives

As further additives, di-phenyl amine and di(C1 to C16 alkyl)penylamines, e.g. octylated/butylated di-phenyl amine, are particularlysuitable as anti-oxidants.

Instead of substituted phenyls, unsubstituted or C1 to C16alkyl-substituted naphtyl moietys can also be used.

Composition of Operating Agent

The operating agent composition generally comprises between 1 and 25% byweight of lubricant—however, this parameter can also be outside of therange indicated, depending on the type of refrigerating machineconcerned—preferably at least 40% by weight, preferably at least 80% ofweight of the additives to the operating agent consisting ofpolyalkylene glycols and/or neopentyl polyols, based on all theconstituents of the operating agent.

The proportion of the particularly preferred polyalkylene glycols withat least one aromatic group is preferably at least 20% by weight,particularly preferably at least 40% by weight, in particular at least80% by weight, based on the proportion of lubricant (i.e. the lubricantswithout refrigerants and additives) in the operating agent composition.

When using lubricant mixtures of different compound classes, theproportion of neopentyl polyol ester used as lubricant is preferably 20to 60% by weight, particularly preferably 40 to 60% by weight, based onthe proportion of lubricant in the operating agent composition in eachcase.

Miscibility

With respect to the overall degree of effectiveness, an advantageoussolubility behaviour between oil and CO₂ is desirable. The behaviour ofCO₂ regarding the solubility properties is highly variable.

The polyalkylene glycols used in the compositions according to theinvention are preferably miscible (soluble) for higher proportions bymass of lubricant in CO₂ over the entire temperature range from thecritical temperature T_(k) to less than −40° C. and in some cases toless than −55° C. With lower proportions of lubricants, thesepolyalkylene glycols are no longer or only partially miscible (soluble)with liquid carbon dioxide.

Investigations of air conditioning circuits operated with CO₂ have shownthat, due to the high miscibility of polyol ester lubricants, such aspentaerythritol esters in particular, a correspondingly high solubilitycan be achieved.

Connected therewith, a dramatic decrease in the viscosity can take placein the region of the driving gear parts, to be lubricated, of thecryogenic compressor. Under the conditions prevailing there, immiscibleor less satisfactorily miscible lubricants such as e.g. mineral oils,polyolefins, alkyl benzenes or even polyalkylene glycols, on the otherhand, do not exhibit the above-mentioned decrease in viscosity. However,as a result of the unsatisfactory miscibility, problems arise regardingthe oil return transportation, particularly in the expansion valve andevaporator components as well as the suction line, particularly at lowflow rates. On the one hand, one of the requirements is to achieve acorrespondingly high mixture viscosity in the compressor, i.e. in theclearance filled by the oil film, on the other hand, a miscibility mustbe guaranteed at low temperatures in the range of the evaporator andsuction line components to guarantee the oil return and a good thermaltransfer as well as a good controllability of the system.

A so-called partial miscibility, i.e. a miscibility gap existing withina certain temperature range for certain mixing ratios, is of greatinterest here. Due to the advantageous temperature/solubility behaviour,refrigerating machines can be used in this case which operate without anoil sump or oil recycling.

Preferably, the lubricant according to the invention exhibits a completemiscibility with the operating agent in the concentration range betweengreater than 0 and 20% by weight, preferably greater than 0 and 5% byweight of the lubricant in the refrigerant at temperatures of 15° C. andless (as low as −40° C., preferably −55° C.) and in the concentrationrange of 30 and 60% by weight in the relevant temperature range of −40°C. (or −55° C.) to +30° C. Outside these ranges, i.e. between greaterthan 5 and lower than 30% by weight, greater than 20 and lower than 30%by weight of lubricant in the refrigerant, a miscibility gap ispreferably present.

The above-mentioned criterion is e.g. that of polyalkylene glycolsclosed by terminal C1-C4 alkyl groups produced by using startingalcohols exhibiting aryl groups. Examples in this respect are cresols,p-hexyl phenol or (hydroxymethyl) benzene. Such polyalkylene glycols aredefined in further detail in the sub-claims.

Experimental Part

The proven methods for testing the wear behaviour and the load bearingcapacity of refrigerating machine oils, e.g. the Shell® “Four ballapparatus” (FBA), the “Almen Wieland testing machine” and the “Falex pinand Vee block test” are suitable only to a limited extent since theinfluence of compressed CO₂ cannot be simulated in this case.

Wear tests carried out with CO₂ at 1 bar did not give any directindications of a strongly negative effect of CO₂ on the wear behaviour.Investigations with the block-on ring” testing machine at 10 bar CO₂, onthe other hand, have shown a distinct influence of the basic oils and,in particular, of additives, on the wear behaviour (D Drees; J Fahl; JHinrichs; “Effects of CO₂ on Lubricating Properties of Polyolesters andPolyalkylene Glycols”; Proc. 13^(th) Int. Colloq. Synth. Lubricants andOperations Fluids, Esslingen 2002, Publication in preparation).

In contrast to test runs with conventional refrigerating machine oils,the bearings exhibited an excellent wear profile after experiments withthe compositions according to the invention.

To assess the long term lubrication properties under the influence ofcompressed CO₂, useful life tests were carried out in specificallydesigned anti-friction bearing test stands under near practicalconditions. Finally, several development products were tested inprototype compressors and test facilities. In a long term test stand,the useful life actually achieved was possible under specific operatingconditions under a CO₂ atmosphere of axial cylinder roller bearings atspeeds up to 8000 min⁻¹ under a CO₂ pressure of 50 bar at a maximumtemperature of 90° C. The axial load was 8 kN.

The basis of calculation commonly used for dimensioning of bearingstakes into account neither the influence of different basic fluids northe effect of additives but is based mainly on the mixture viscosity.Important influential factors such as the presence of gases, e.g. CO₂ inthis particular case, are not included in this calculation even thoughthese play an important part. In order to obtain details in thisrespect, tests of the useful life are necessary under near practicalconditions.

The test parameters are selected in such a way that an optimum testperiod is achieved for the investigation. The test parameters aresummarised in Table 1.

The axial loading of the axial cylinder roller bearings to be examined(geometry AXK 18×35×4.5) is effected via cup-spring assemblies and canbe adjusted by means of separator disks of different thickness. The testis carried out until at least one bearing fails as a result of beingdamaged. The test parameters are as follows:

TABLE 1 Tests parameters Abbreviation Parameter (unit) Value Axialloading Pa (N) 8000 Hertz pressure (roller) Pmax (N/mm²) 1622 Speed N(min⁻¹) 800 CO₂ pressure P (bar) 50 Oil temperature T oil (° C.) 90

The oils shown in table 3 were tested. ND 8 is a commercial product fromthe Japanese compressor manufacturer NIPPONDENSO (manufactured byIdemitsu Kosan) with, among other things, approximately 1-2% by weightof tricresyl phosphate and 0.5% by weight of BHT(2,6-di-tert.butyl-4-methyl phenol) added. SP10 and SP 20 are commercialproducts from the Japanese compressor manufacturer SANDEN (alsomanufactured by Idemitzu Kosan) with similar additives.

The polyalkylene glycol lubricating oils preferably show (P4), evenwithout an addition of phosphoric acid esters a lubrication behaviourcorresponding to the terminally methylated polyalkylene glycol added(compare PAG—oil ND 8). The results in table 2 clearly show that theclaimed addition in combination with the claimed basic liquidsconsiderably prolongs the useful life under the effect of compressedCO₂. This effect is particularly noticeable also in association withhighly soluble neopentyl polyol esters.

For the application of CO₂ in motor cars, axial piston machines arepreferred because of their compact design and the homogeneous conveyingstreams. During initial endurance tests with prototype compressors, thelubrication of the roller bearing subject to extreme stress, inparticular, proves to be problematic. The test runs with the commercialpolyol esters and polyalkylene glycol oils resulted in much shorteruseful life. As a result of the favourable solubility characteristicsand the excellent load bearing capability under the influence of nearcritical CO₂, the claimed formulations are suitable for use as highperformance lubricants for CO₂ motor car air conditioning and heat pumpsystems.

In the case of the phosphate ester additives which are easilycommercially available, a distinction is made between those comprisingcresol (additive C in Table 3) and those comprising xylenol groups (usedrarely). The subject matter of the invention consists of t-butylated(additive A in Table 3) and/or isopropylated (additive B in Table 3)triphenyl phosphates. Surprisingly enough, these have proved to be muchmore suitable than conventional tricresyl phosphate or triphenylphosphate.

TABLE 2 Basic oil measurement values with and without additive Startingcomponents Molecular Kinematic Starting weight Density viscosityViscosity Pour Useful life L Name Type Monomer alcohol/ End (g/mole)(kg/m³) (mm²/s) index point (h) Added (h) Polyether EO:PO group group(ca.) 15° C. 40° C. 100° C. — (° C.) — A B C ND8 PAG 0:1 Me Me 930 99242.3 9.2 212 −36 61 SP PAG 1:1 Me Me 1300 1019 100.9 19.8 221 −45 105 20SP PAG 1:1 Me Me 900 998 47.6 10.2 210 −45 90 20 P1 PAG 0:1 Butanol OH930 990 58.9 11.4 191 −45 46 156 134 52 P2 PAG 1:1 Me Me 1015 1038 59.413.4 235 −51 54 183 167 61 P3 PAG 0:1 Furfurol Me 930 993 41.3 9.4 219−51 31 202 156 85 P4 PAG 1:1 Phenol Me 940 995 44.0 7.3 129 −42 60 492380 144 P5 PAG 2:1:1 Butanol OH 772 981 47.2 11.5 248 −57 40  (66) 80 64THF:EO:PO Ester Alcohol Acid E1 POE DPE i-C_(g) 1150 974 170.0 17.2 108−30 22 194 204 71 E2 POE DPE n-C_(g)/ 730 1006 80.0 9.9 105 −39 17 161153 45 n-C DPE = Dipentaerythritol, ND8, SP 20 and SP 10 are commercialproducts with several additives from Nippondenso and Sanden, all othersamples comprise no additive, unless otherwise indicated.

TABLE 3 Additives # Additive P Content A. t-butylated triphenylphosphate 8 B. isopropylated triphenyl phosphate 8 C. tricresylphosphate 8.4

1. Operating agent composition comprising (A) carbon dioxide asrefrigerant, (B) polyalkylene glycols and/or neopentyl polyol esters aslubricant and (C) a phosphate ester with the following structure:

wherein R optionally, identically or differently for each of the threephenyl moieties and optionally, identically or differently for each n,represents H one or more C1 to C6 hydrocarbon moieties and n optionallyidentically or differently for each of the three phenyl moietiesrepresents an integer of 1 to 5, with the proviso that for at least oneof the three phenyl moieties R is t-butyl and/or isopropyl.
 2. Operatingagent composition according to claim 1 comprising said phosphate esterin a quantity of 0.1 to 3% by weight, based on the lubricant. 3.Operating agent composition according to claim 1 characterized in thatsaid polyalkylene glycols comprise no free hydroxy groups.
 4. Operatingagent composition according to claim 1, characterized in that saidoperating agent composition comprises polyalkylene glycols which, basedon the polymer chain and the alkylene oxide monomer units used, consistsof essentially exclusively monomer units of the type —(—CH(CH₃)—CH₂—O—)—or —(—CH₂—CH(CH₃)—O—)—, 20 to 80% monomer units of the type—(—CH(CH₃)—CH₂—O—)— or —(—CH₂—CH (CH₃)—O—)— and for the remainingresidue of monomer units of type —(CH₂—CH₂—O—)— or 20 to 80% monomerunits of the type —(—CH(CH₂CH₃)—CH₂—O—)— or —(—CH₂—CH(CH₂CH₃)—O—)— andfor the remaining residue of monomer units of type —(—CH₂—CH₂)—O—)—. 5.Operating agent composition according to claim 1, characterized in thatsaid operating agent composition comprises polyalkylene glycols and/ortheir mixtures having a number average molecular weight of 200 to 3000g/mole.
 6. Operating agent composition according to claim 1,characterized in that said polyalkylene glycols comprise aryl groups orheteroaromatic groups which may optionally be substituted with linear orbranched alkyl groups or alkylene groups.
 7. Operating agent compositionaccording to claim 1, characterized in that said polyalkylene glycolshave the following end groups -alkyl, aryl, alkylaryl, aryloxy, alkoxy,and/or alkylaryloxy end groups having 1 to 24 carbon atoms.
 8. Operatingagent composition according to claim 1, characterized in that saidoperating agent composition comprises esters or an ester mixture,wherein said esters are obtainable by reacting neopentyl polyols, withlinear and/or branched C4 to C12 carboxylic acids, optionally with anaddition of C4 to C12 dicarboxylic acids.
 9. Operating agent compositionaccording to claim 1, characterized in that the operating agentcomprises neopentyl polyol esters and polyalkylene glycols. 10.Operating agent composition according to claim 1, characterized in thatsaid operating agent composition comprises at least 10% by weight ofsaid polyalkylene glycols and said neopentyl polyesters, based on allthe constituents of said operating agent.
 11. Operating agentcomposition according to claim 1, characterized in that said operatingagent consists predominantly, apart from said phosphate esters and saidrefrigerant, of said polyalkylene glycols and said neopentyl polyesters;based on the proportion by weight.
 12. Operating agent compositionaccording to claim 1, characterized in that the operating agentadditionally comprises a diphenyl amine, a di(C1 to C16 alkyl) phenylamine as antioxidant and/or a diphenyl amine in which one or two phenylgroups have been exchanged for naphthyl groups.
 13. Operating agentcomposition according to claim 1, characterized in that said phosphateester has at least for one of said phenyl moieties, an R which istert-butyl and/or isopropyl.
 14. Operating agent composition accordingto claim 1 for use in refrigerating machines.
 15. Operating agentcomposition according to claim 1 for use in freezing equipment havingevaporation temperatures of less than −30° C., wherein lubricants areused which comprise more than 90% by weight of neopentyl polyol esters.16. Operating agent composition according to claim 1 for use in airconditioning equipment of cars, wherein lubricants are used whichcomprise more than 90% of polyalkylene glycols.
 17. Operating agentcomposition according to claim 5, wherein said polyalkylene glycolsand/or their mixtures have a number average molecular weight of 400 to2000 g/mole.
 18. Operating agent composition according to claim 6,wherein the alkyl groups or alkylene groups have a total of 1 to 24carbon atoms.
 19. Operating agent composition according to claim 8,wherein said neopentyl polyols comprise pentaerythritol,dipentaerythritol and/or tripentaerythritol.
 20. Operating agentcomposition according to claim 11, wherein said operating agent consistsexclusively, apart from said phosphate esters and said refrigerant ofsaid polyalkylene glycols and said neopentyl polyesters.
 21. Operatingagent composition according to claim 14 wherein said refrigeratingmachine is in a motor vehicle.